'89 Century Wagon - Reverse lights inop... please help.

I can't afford to take this car into the shop and get it diagnosed. So, anything anyone can tell me, I really appreciate it.

Recently the reverse lights have stopped working. On the odd time, when I put it in reverse, the lights will work. When I put it back in drive and then back in reverse again, it doesn't work, again.

Now, I haven't looked into anything as I'm assuming the worst and that it needs a switch.

It's a V6 3.3 litre engine, 4 speed.
I've been having trouble finding parts for this car in the past, but I've found a large parts company in Wisconsin that sell discontinued parts at reasonable prices and ship by Fed-Ex.

Is it a fuse?
Is it the switch?
Is it where the switch is located (which I don't know where that is)?
Is it wiring?
And, I hate to even say this, but is there a chance it has anything to do with the water leak I'm getting in the 2nd row floor when it rains?

Thanks again for your help and ideas.